Remarkable Section near Korolevu Hill.—Between the hills of Korolevu and Nganga-turuturu, at an elevation of about 300 feet above the sea, there is a singular exposure of tuffs horizontally stratified and forming a low escarpment or line of cliff about 15 feet high on the hill-side. These beds display the passage from basic tuffs below to relatively acid tuffs above, and they establish that in this locality the period of acid andesites followed that marked by the eruption of basalts and basaltic andesites. From their horizontal and undisturbed position, it may be inferred that these deposits began to be formed under the sea when the activity of the submarine basic vents was on the wane. In their composition and in the various degrees of coarseness of their materials, we can plainly discern the history of volcanic action in this locality.
A hard compacted palagonite-tuff makes up the lower half of the thickness of beds exposed, 15 feet in all. The greater portion of it has the uniform texture of a sedimentary rock, fine-grained below where the fragments are ·1 to ·3 mm. in size, and becoming coarser above where the larger measure 1 to 2 mm. It is composed of more or less angular fragments of a basic vacuolar isotropic glass, and of plagioclase and augite with much fine palagonitic débris. There is no effervescence with an acid; but in the upper part there are a few casts of foraminifera of the “globigerina” type, as indicated in the thin sections. Above this lies a bed of a similar basic tuff, having however a banded appearance from the arrangement of materials of different degrees of coarseness, the finer being ·1-·2 mm. in size, the coarser ·4-·8 mm. There is little or no carbonate of lime; but occasional tests of foraminifera of the type above mentioned occur in the slide. The basic tuffs here abruptly terminate. They represent the quiet deposition in water comparatively deep of the products of marine erosion, and of the finer ejectamenta of some distant subaerial vent.
Above the basic tuffs lie a series of tuffs, about 5 feet in thickness, and composed mainly of the debris of acid andesitic rocks of the hornblende-andesite type, such as occur in the Ndrandramea district. They mark a period of active eruption on the part of some neighbouring acid andesitic vent in this neighbourhood, which the subsequent explorer may be able to identify with some volcanic “neck.”
These tuffs are composed partly of fragments of a hemicrystalline hornblende-andesite and partly of crystals, broken and entire, of plagioclase, hornblende, rhombic pyroxene, and augite. The plagioclase is tabular, zoned, and glassy, and gives extinctions of oligoclase-andesine (6 to 12°). The hornblende is bottle green, markedly pleochroic, and gives extinctions up to 14°. The rhombic pyroxene has the characters described on page [301], in the case of the Ndrandramea rocks. The augite is less frequent, but the two pyroxenes are sometimes associated as intergrowths.
These acid tuffs do not effervesce with an acid, nor can any tests of foraminifera be observed in them; but since these organisms are represented in the basic tuffs below, it is highly probable that the whole series of these horizontal beds is submarine. The first or lowest bed of the acid tuffs indicates a somewhat violent volcanic outbreak in this neighbourhood, following the deposition of the basic tuffs. It is composed of loosely compacted subangular fragments, 1 to 3 millimetres in size, in which the macroscopic prisms of the rhombic pyroxene are especially frequent. It passes upward without interruption into a regularly grained sandstone formed of rounded and subangular fragments measuring ·3 to ·7 mm. across. Above this lies a quite distinct bed, a few inches thick, of a fine compact clay rock, where the mineral fragments measure only ·05 to ·12 mm. in diameter, hornblende being well represented, although the rhombic pyroxene is very scanty. Up to this time these beds of acid tuffs indicate a gradual defervescence of the volcanic activity that began with some violence, as shown by the characters of the lowest bed. Now another outbreak occurred, and overlying the clay-like bed we find a coarse tuff made up of fragments 2 to 5 millimetres across, and approaching in texture and appearance a subaerial tuff, but in other respects similar to those below it. It is the last and uppermost of this series of acid tuffs, and with it terminates an interesting record of the past in this region, the chief features of which may thus be summarised.
A prolonged period of quiet deposition of submarine basic tuffs, the products partly of marine erosion and partly of distant eruptions, was abruptly followed by the outbreak of a neighbouring vent during which tuffs formed of the debris of acid andesites were deposited. The gradual decrease in the degree of activity is plainly shown in the gradual diminution in size of these tuffs, until they acquire the fineness of a clay. Then another burst of activity from the same vent or vents occurred, and the record ends. Since that time there has been apparently an upheaval to an elevation of 300 feet above the sea. As, however, the beds are quite undisturbed, the emergence may have been due to the lowering of the sea-level, a subject which is discussed in [Chapter XXVII].
Coast between Wailea Bay and Lekutu.—The hills here often approach the coast, their spurs running down to the beach. In the low range, 250 to 300 feet high, east of Wailea Bay, are exposed palagonite-tuffs dipping gently north-east and composed of fragments of a vacuolar basic glass, more or less palagonitised, and of minerals (plagioclase, etc.) not exceeding 2 mm. in size. These deposits are apparently non-calcareous and show no organic remains.
Farther along the coast towards Nativi basic tuffs and agglomerates appear at the surface; but the underlying rock, exposed in position in the stream-courses and prevailing along much of the sea-border to Nativi and a mile or so beyond, is a vesicular semi-ophitic basaltic andesite with coarse doleritic texture and containing much interstitial smoky glass. (It belongs to the non-porphyritic group of genus 9 of the augite-andesites described on page [273].) Such rocks evidently represent ancient flows. They give place as one proceeds east to porphyritic semi-ophitic doleritic rocks of the same genus and to semi-vitreous basic rocks. About half a mile west of Nukunase a vesicular doleritic basaltic andesite forms a spur protruding at the coast. It is semi-ophitic and contains in the smoky glass of the groundmass little irregular cavities filled with a yellowish residual magma like palagonite in character. (It is referable to genus 12 of the augite-andesites, described on page [275].) A few paces west of this spur a vertical dyke, 20 feet wide and trending N.W. and S.E., appears on the beach. It is formed of a bluish scoriaceous basaltic andesite containing much glass in the groundmass and showing imperfectly developed felspar lathes. It is included in genus 4 of the augite-andesites described on page [270].
A little east of the spur there is another dyke apparently vertical and formed of a vesicular rather than a scoriaceous basaltic andesite referred to genus 1 of the augite-andesites (page [267]). It differs from the rock of the previous dyke in the presence of small plagioclase phenocrysts which contain abundant magma-inclusions; but it resembles it in the characters of the groundmass. This dyke is about 40 feet in thickness and trends N.E. and S.W.
It may be inferred from the foregoing remarks that there was at one time a volcanic vent in the district west of Nukunase. The lines representing the trend of the two dykes above noticed would if extended meet at a common focus a little way inland. The rocks of the dykes differ conspicuously from the prevailing doleritic rocks that form, as before remarked, the ancient flows, the average length of the felspar-lathes in the former being ·1-·2 mm., in the latter ·3-·4 mm. Both, however, belong probably to the same vent of which now the exact situation would not be easy to discover, on account of the re-shaping of the surface through the denuding agencies.